Various methods of providing security documents, such as negotiable instruments, with tamper evident systems to prevent alteration have been proposed. More specifically, pressure-sensitive autogenous coatings comprising color developer and encapsulated color former have been used to verify the authenticity of security documents. Once an external pressure is applied to the coating, microcapsules rupture and release the color former which then reacts with the color developer to form a colored image. This autogenous coating may be present on the front side, the rear side, or both sides of the document. After the important information is entered on the front side, the security document could later be tampered by one with ill intention. The use of a chromogenic coating thus provides a safeguard against tampering. For example, if a document is altered, the microcapsules in the altered area will rupture, causing release of the color former to react with the color developer. This unanticipated coloration displays a sign of tampering. If a document is duplicated by a copy machine, its authenticity may be determined by applying external pressure to the autogenous coating to form a visible colored image. The appearance of such instant in situ image confirms that the document is authentic.
U.S. Pat. No. 4,846,502 to Chang et al. discloses that a first autogenous coating may be applied on the front surface of a document bearing a principal image. A second localized autogenous coating is additionally applied on the rear surface of the document directly opposite and at least generally coextensive with the first coating to provide double protection. Any attempt to alter the imprinted visible indicia on the autogenous coating of the front side will simultaneously form a warning color in both front and rear coatings. Due to its autogenous nature, the rear side coating develops the mirror image of the indicia once such entry is initially made onto the front side coating. This not only permits the imprinting of a mirror image of the monetary amount, for example, as in the case of postal money order, but also serves as a tamper evident device for the principal or mirror image.
The Chang '502 patent also reveals there may be a localized carbon transfer medium on the front side of a lower transfer sheet, which is adjacent to the rear side autogenous coating and may produce a mirror image in the form of a carbon black on top of the rear side autogenous coating. Regardless of how it is formed, this carbon black image serves as an irrevocable reference. If it coincides with the front side image, it validates the authenticity of the negotiable instrument. On the other hand, if these images differ because of forgery, it suggests that the original imprinted visible indicia have been falsified. However, the tamper evident system of the Chang '502 patent always includes an autogenous coating which is in the form of a latent message, such as VOID, underneath the principal image on the front side of the document and has a second localized autogenous coating at least directly opposite the area of the principal image on the rear side of the document. Otherwise, an additional lower sheet having a localized carbon transfer medium or an inked typewriter ribbon is required to produce the mirror image. For practical purposes, this system is applicable for documents in which the second localized autogenous coating can be predetermined, such as a postal money order, a cashier's check, checks, certificates, preprinted documents, etc.
U.S. Pat. Nos. 5,395,138 and 5,605,873 to Chang disclose a localized pressure-sensitive autogenous system to verify the authenticity of a negotiable instrument, such as a bank check. Thus, when the check is submitted for payment at a bank, for example, the teller can verify the authenticity of the check by simply applying external pressure, such as by use of a fingernail or stylus, to the verification area conveniently located adjacent the endorsement area of the check before cashing the check. The autogenous spot coating may be located anywhere on the rear side of the document and does not need to be directly opposite the principal image on the front side.
The article "Security Document", Research Disclosure No. 187, November 1979, pp. 638-639, reports that by incorporating pressure-sensitive copy image forming means on the rear surface of a security document, the external pressure to imprint indicia on the front surface of the document also causes the formation of additional immediate image on the rear side of the document. If the indicia are subsequently lifted off using a correctable typewriter ribbon and different indicia are entered, the alteration will be visibly apparent from the image on the rear side, since the resultant double image is easily recognizable. The coating may be over the entire rear surface of the document or in the verification areas. This double image formation is caused by rupture of image-forming material from the microcapsules.
Autogenous copy systems have been widely used in many applications, such as business forms, sales receipts, duplicate of checks, pressure-sensitive labels, and the like. In these applications, the coating is on the front surface so that the image developed by the external pressure serves as the indicia for the document. Use of an inked ribbon is not required to transfer images onto the self-imaging coating. Since the coating is "live" at all times, any unintended pressure which may rupture the microcapsules, such as scratching by a hard object, folding, writing pressure when inadvertently placed underneath another substrate, etc., will develop unwanted image in the principal image areas.